There isn't
really anything as "too big water change" when it comes to goldfish, the more you change, the better

One of my books recommend 75% every second week or ideally, 50% each week. I follow the earlier mentioned since the nitrates don't manage to reach any mentionable levels during the first week. But it all come down to your schedule and habits, whichever way you choose is fine, the fish will get used to it soon enough C:
But I'm getting a little confused, in one thread you say your tank is 50 gallons, and now it's slightly more than 100? How does it work? : D